Another possibility is to make a hartman mask for the lens. This method is used to find infinity focus for astrophotography. Use this link to create the dimensions and position of the holes: http://www.billyard-ink.com/Hartmann.shtml

In use, you'd point it at the night sky so that you'll see trails of stars. Slowly rack the focus and take a long exposure to test that setting. As you approach infinity, the two star trails will merge into one line.

Be sure to look at a single and not a double star!

Once you find focus, mark that point and note the local temperature as this will change depending on the materials the lens has and its focal length.

You may want to bring some gaffer tape to secure the focus ring once you've found the right spot so that it won't move if you touch the camera.

The problem with the Bahtinov mask with camera lenses is that it gets very hard to see the spikes. I've tried these with a 200mm and 300mm focal lengths and it's quite difficult. It gets better if you have a computer screen to see the image at 100%. The Hartman mask works down to 50mm focal lengths. It's best to orient the Hartman mask so that it's at right angles to the star trail to see the gap in the lines.
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smigolApr 25 '12 at 19:45

Interesting, I've never had a problem with my Bahtinov on my 50mm.
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rfuscaApr 25 '12 at 19:52